Without limiting the scope of the present invention, its background is described with reference to testing hydrocarbon formations, as an example.
It is well known in the subterranean well drilling and completion art to perform tests on formations intersected by a wellbore. Such tests are typically performed in order to determine geological or other physical properties of the formation and the chemical and physical properties of the fluids contained therein. For example, parameters such as permeability, porosity, fluid resistivity, temperature, pressure and bubble point may be determined. These and other characteristics of the formation and fluid contained therein may be determined by performing tests on the formation before the well is completed.
One type of testing procedure is to obtain a fluid sample from the formation to, among other things, determine the composition of the formation fluids. In this procedure, it is important to obtain a sample of the formation fluid that is representative of the fluids as they exist in the formation. For example, the sample is used to determine the economic value of fluids within the formation. In addition, the composition of the formation fluids is used to determine the type and capacity of the processing equipment required to process fluids extracted from the formation.
In the past, sampling of formation fluids was accomplished by collecting a large volume of fluid through the drill string which may be on the order of thousands of gallons of formation fluids. This type of large scale sampling is, however, timely and expensive. In an alternative sampling procedure, formation fluids may be sampled on a smaller scale by lowering a sampling tool into the wellbore on a wireline, slick line or tubing string. In this case, when the sampling tool reaches the desired depth, one or more ports are actuated from the closed position to the opened position to allow collection of the formation fluids. The ports may be actuated in variety of ways such as by electrical, hydraulic or mechanical methods. Once the ports are opened, formation fluids travel through the ports and a sample of the formation fluids is collected within a chamber of the sampling tool. After the sample has been collected, the sampling tool may be withdrawn from the wellbore so that the formation fluid sample may be analyzed.
It has been found, however, that with the use of conventional formation sampling tools, the fluid sample is obtained relatively quickly which can cause phase change degradation of the formation fluid due to flashing as the fluid flows into the sampling chamber. This phase change degradation may result in irreversible chemical and physical changes in the formation fluid. For example, in a typical sampling procedure, the formation fluids flow through one or more valves or passageways to enter the sampling chamber. The inherent pressure drop across the valves or passageways creates the possibility that lighter fractions present in the sample will flash, or come out of solution, during collection. Once flashing has occurred, the resulting sample may no longer be representative of the fluids present in the formation.
It has also been found that as conventional formation sampling tools are retrieved from the wellbore, the reduction in hydrostatic pressure acting on the sampling tool may result in a reduction of the fluid pressure within the sampling chamber. This drop in pressure may similarly cause phase change degradation of the sample as the sampling tool is removed from the wellbore. In the past, it has been difficult to know whether the sample has undergone phase change degradation either during collection or retrieval from the wellbore. As such, it has been difficult to determine whether the sample is representative of the fluids present in the formation.
Therefore, a need has arisen for an apparatus and method for obtaining a fluid sample from a formation without phase change degradation of the sample during collection or retrieval of the sampling tool from the wellbore. A need has also arisen for such an apparatus and method that is capable of verifying whether the sample has undergone phase change degradation.